(1) Field of the Invention
The present invention relates to a display panel used for displaying images, such as for a computer or television, especially a display panel with silver electrodes formed on the panel surface, and to a production method for such a display panel.
(2) Description of Prior Art
In the field of image displays for computers, televisions and other devices, recently field emission display panels, plasma display panels (PDP) and other types of display panel have received increasing attention as devices that allow a large color image display in a thin package. The PDP especially, because of its excellent high speed response and wide viewing-angle characteristics, has become the object of-great activity, as companies and research institutions step up R&D efforts aimed at a mass market.
A PDP has a front glass substrate and a back glass substrate, separated by barrier ribs. A plurality of display electrodes are formed in a stripe pattern on the back of the front glass substrate (the side facing the back glass substrate), and a dielectric layer is formed covering the electrodes.
In a conventional PDP, the front glass substrate is made from a soda-lime-borosilicate glass sheet, and the display electrodes are Cr—Cu—Cr or silver, which are relatively easily formed.
A silver electrode can be formed by thin-film method, but the relatively low-cost thick-film method is used also. The first step in the thick-film method is to form a thick silver film in the shape of the electrode pattern, by applying a silver paste containing silver particles, glass frit, resin solvent and such to the front glass substrate by a screen printing process, or by affixing a film containing silver particles, glass frit, resin and such by a lamination process, for example. Patterning is followed by baking at over 500° C., in order to remove the resin contained in the paste or film and to fuse the silver particles and glass frit. Fusing of the fused silver particles raises their conductivity, and fusing of the glass frit affixes them to the front glass substrate.
After baking, the dielectric layer is formed. Powder from ground low-melting lead glass, resin, and solvent are mixed to form a past, which is applied by screen-printing or lamination to cover the silver electrodes. When the solvent has dried, the panel is baked at over 500° C. a second time. At high temperature, the resin in the paste is removed and the low-melting lead glass is fused, forming the dielectric layer.
By the same processes, electrodes and a dielectric layer are formed and affixed to the back glass substrate as well.
In a PDP which uses silver electrodes, silver is ionized in the baking process and diffused inside the glass substrate, by reactions such as ion exchange with sodium included in the glass (usually 2.5 wt % to 15 wt %). This diffusion of silver is thought to proceed in proportion to the temperature and duration of baking. The diffused silver is reduced inside the glass substrate, forming colloid and causing yellowing of the glass. Yellowing in the front glass substrate is especially problematic, as it can cause deterioration of the color temperature and loss of image quality.
To reduce yellowing, suppression of silver ion diffusion by lowering baking temperature has been considered, but decomposition of the resin and softening of the electrode and dielectric layer materials are also dependant on the baking temperature, making change difficult. Similarly, reduction of baking time has been considered also. However, by simply shortening the baking time, resin may be left in the electrodes and dielectric layer, and fusion in these parts may be insufficient, carrying the risk of reduced electrode conductivity and reduced dielectric layer insulation.
This yellowing phenomenon occurs not only in PDPs, but also in field emission display panels and other display panels which have thick-film silver electrodes formed on a glass substrate, creating high demand for technology to suppress yellowing.